The present invention relates to mercury dispensing compositions.
The compositions of the invention are particularly suitable for the use in dosing mercury inside fluorescent lamps.
As known, fluorescent lamps require for their operation a gaseous mixture at pressures of some hundreds of hectoPascal (hPa), formed by noble gases and mercury vapors. In the past mercury was introduced into the lamps in liquid form, either by direct dripping into the lamp, or inside of small glass vials which afterwards were opened inside the lamp. However, due to the toxicity of mercury, the most recent international regulations have imposed the use of the lowest possible quantity of the element, compatibly with the lamps functionality; this has rendered the liquid dosage methods obsolete, because these are not capable of dosing in lamps quantities of mercury of few milligrams or even smaller than one milligram.
Another method for the introduction of mercury into lamps is by means of metal amalgams. However, this method implies a problem: some manufacturing steps of the lamps are carried out at relatively high temperatures, generally higher than 400° C., when the lamp is not sealed yet, while the mercury release from these materials starts already at low temperatures, between about 100 and 300° C. depending on the metal with which mercury is amalgamated; in these conditions emissions of mercury, which is a harmful metal for health, occur into the working environment.
In order to overcome these problems, it was proposed in the past the use of various solid products which allow to overcome or at least reduce the problems seen before.
U.S. Pat. No. 3,657,589 in the Applicant's name discloses TixZryHgz compounds, which do not release mercury when heated up to about 500° C., but can release it when heated to about 800-900° C. (so-called activation treatment); the preferred compound of this family is Ti3Hg, sold under the trade name St 505. These compounds have the advantage that they can be powdered and dosed into small weight quantities for producing mercury dispensing devices containing the required amount of this metal. A problem of these compounds is, however, that they undergo a partial oxidation during the lamp manufacturing steps, whereby the amount of mercury released during activation is only about 40% of the total mercury content, which forces to introduce into the lamp a quantity of mercury noticeably larger than necessary, with disposal problems at the end of the life of the lamps.
British patent application GB-A-2,056,490 discloses Ti—Cu—Hg compositions having better properties of mercury release compared to those of the compounds of U.S. Pat. No. 3,657,589. In particular, these compounds are stable in air up to about 500° C., while by heating up to 800-900° C. they release quantities of mercury higher than 80%, or even than 90%.
The U.S. Pat. No. 5,520,560, U.S. Pat. No. 5,830,026 and U.S. Pat. No. 5,876,205 disclose combinations of powders of the compound St 505 with a promoter of the mercury yield (respectively, copper-tin alloys with possible additions of small quantities of other transition elements; copper-silicon alloys; and copper-tin-Rare Earths alloys); the addition of the promoter allows to increase the mercury yield from the compound St 505 up to values of 80-90%, even after its oxidation, thus avoiding the need of using a large excess of mercury as happens with the compound St 505 used alone.
Finally, U.S. Pat. No. 4,464,133 proposes to use mixtures of powders of the compound Ti3Hg with an element selected between nickel or copper; according to what is stated in this document, by these mixtures it is possible to achieve the mercury release already at the temperature of 770° C.
The releasing of mercury from these mixtures and compositions is normally obtained by heating by means of radiofrequencies, by positioning an induction coil externally to the lamp in a position close to the device which comprises the mercury containing material; good yields of the metal are achieved by heating treatments of total duration of about 20-30 seconds per lamp.
However, the properties of mercury releasing from known compositions and mixtures, although good, are not yet completely satisfactory for lamp manufacturers. An optimal mercury dispenser for lamp manufacturing should have the following features:                zero metal emissions up to at least 500° C., and possibly up to about 600° C., for being used also in the manufacturing of circular lamps, wherein some operations require higher temperatures than in the case of linear lamps;        total or almost total yield of mercury so that, for the same quantity of mercury released in the lamp, the initial amount of mercury present in the device is the lowest possible, to comply with international regulations on the use of harmful materials in industrial manufacturing;        an activation temperature lower than those used hitherto, to reduce the energy consumption in the manufacturing line (the induction coils have to be provided with a lower power);        shorter activation times with respect to those required by the compositions used hitherto, to increase productivity.        